1. Field of the Invention
The present invention relates to an exhaust device for a motorcycle including an engine having a plurality of cylinders.
2. Description of Background Art
In a conventional exhaust device for a motorcycle including an engine having a plurality of cylinders, an exhaust control valve is provided in an exhaust passage extending from a plurality of exhaust pipes connected to exhaust ports for the cylinders to an exhaust muffler, thereby controlling a sectional area of the exhaust passage. The exhaust control valve is provided to block valve operating noises generated from the cylinders and also to obtain exhaust control characteristics according to engine speed. The exhaust control valve is a so-called butterfly valve rotatably supported to a pivot shaft so that a main surface of the butterfly valve is rotated about the axis of rotation of the pivot shaft to adjust the degree of opening of the exhaust passage.
Further, it is known that a separator is provided on the upstream side of the exhaust control valve at a portion of the exhaust passage where two exhaust pipes are united to one exhaust pipe to join the flows of exhaust gas. The separator functions to gradually join the flows of exhaust gas from the two exhaust pipes toward the one exhaust pipe.
In the fully open condition of the exhaust control valve, the main surface of the exhaust control valve is substantially parallel to the flowing direction of the exhaust gas in the exhaust passage. Further, the main surface of the exhaust control valve in its fully open condition is substantially perpendicular to a main surface of the separator. See, for example Japanese Patent Laid-open No. 2005-83358.
When the exhaust gas flows in the exhaust passage in the fully open condition of the exhaust control valve, it is preferable to minimize an exhaust resistance against the flow of exhaust gas in the exhaust passage. However, in the above conventional configuration using the butterfly valve, when the exhaust gas having passed through the separator flows through the butterfly valve, the flow of exhaust gas is hindered by the thickness of the butterfly valve, causing an increased exhaust resistance.